a. Field of the Invention
The present invention relates to risers for use in the extraction of hydrocarbons and in particular to risers that are used to extract oil or gas from offshore and deepwater fields.
b. Related Art
Risers are high pressure dynamic tubular structures used in the extraction of oil and gas from offshore fields. They extend from the seabed to the surface production vessel and are used to transport oil, gas and injection fluids.
In deep water (for example a depth of greater than 1000 meters) there is often a limited number of feasible riser solutions for a particular field development. This is due to the many design, operational, commercial and contractual constraints. This limitation is particularly evident on developments in ultra deep water (a depth of typically between 1500 and 3000 meters) which typically require a large number of risers, utilise dynamic production vessels such as turret and spread moored Floating Production, Storage and Offloading (FPSO) vessels and are often located in an environment that has significant wave, current and wind loading. For these applications there is a demand for improved riser technology and system configurations to assist future developments.
FIG. 1 shows a schematic depiction of a Single Line Offset Riser (SLOR™), which is recognised as a field proven deepwater riser arrangement that has been successfully deployed on two West African projects. The SLOR comprises a near-vertical steel pipe section 2 which is tensioned by a near-surface buoyancy module 3. The connection to the production vessel 1 is made via a compliant, flexible pipe catenary section 4. At the seabed the vertical tension is reacted by a foundation (not shown) that can be either a driven pile, suction pile or gravity base structure.
It is anticipated that the SLOR arrangement will be used on future worldwide deepwater developments. However, the potential for structural clashing between adjacent SLORs requires a large separation to be maintained. FIG. 1 shows schematically that although the vessel 1 may be capable of receiving a significant number of risers it is necessary to provide a separation between the two SLORs shown in FIG. 1. In addition, clearance must be maintained with mooring lines and thus the scope of application of the SLORs is greatly limited to developments in which only a small number of risers is required. This can be a serious limitation on large deepwater projects where 20-30 risers is a typical requirement.
It is known to use near surface buoys to support a plurality of catenary risers, which connect to a respective plurality of flexible catenaries that provide a connection to a surface vessel. Examples of such arrangements can be found in, for example, U.S. Pat. No. 5,957,074 & U.S. Pat. No. 5,639,187.